Roadways, sidewalks, bridges, buildings, water ducts, reservoirs, and other infrastructure and structural components are often manufactured from cementitious composites such as concrete. Concrete includes cement and other things, such as various aggregates and paste. Aggregates include small materials such as sand, gravel or crushed stone. Often, the paste that holds the aggregates together is water and Portland cement. Portland cement is a generic term for the most prevalent type of cement. Cement typically makes up from 10% to 15% of the total mass of concrete. Portland cement is a type of hydraulic cement, which means that when water is added, a chemical reaction is started that causes the cement to harden and set, holding the aggregates together in a rocklike mixture—concrete. Before the concrete is allowed to harden, the concrete mix is poured into a mold so that it will harden into the desired shape. The Portland cement is typically made from a combination of calcareous material (usually limestone or other calcium carbonate-based materials) and argillaceous material (usually siliceous and aluminous minerals containing substantial amounts of clay-like components). A wide range of chemicals are added to concrete that act as plasticizers, accelerators, retardants, dispersants, and water-reducing agents. Called admixtures, these additives can be used to increase the workability of a concrete mixture, the strength of the concrete, the amount of time the concrete will take to harden and achieve full strength, and other desirable properties. The proportions of the various raw materials that go into the concrete must be carefully controlled and measured in order to obtain a finished product with the desired characteristics.
High strength cement-based materials such as macro-defect-free (MDF) cements are being developed for use in many applications that have not been possible with traditional cement and concrete technology. MDF refers to the absence of relatively large voids or defects which are usually present in conventional mixed cement pastes because of entrapped air, inadequate dispersion, and porosity that develops as water soaks into cement particles and aggregate and leaves behind voids. Such voids and defects limit the strength of conventional Portland cement. MDF cement is a polymer-cement composite. The polymer and cement react synergistically to create a unique microstructure with distinct characteristics. The base polymer of the cementitious composite of MDF cement is a water-dissolvable polymer, such as polyvinyl alcohol. High shear mixing and hot press molding processes are typically applied to the mixture during production. MDF cements are characterized by very high flexural strength and a high modulus of elasticity. Flexural strength, also known as modulus of rupture, bend strength, or fracture strength, is a mechanical parameter for a material's ability to resist deformation under load. Modulus of elasticity is a number that measures an object or substance's resistance to being deformed elastically when a force is applied to it. The relatively high flexural strength and high modulus of elasticity of MDF cements are thought to be a result of the elimination of the majority of the voids that are in typical cementitious composites caused by air entrapped during the mixing, and the elimination of the majority of pores and capillaries that are formed when water is desiccated during cement hydration.
In spite of the promising mechanical properties of MDF cement materials, they have not been successfully commercialized because of poor water resistance. The water-soluble polyvinyl alcohol base polymer continues to be hydrophilic even after the initial curing reaction and the subsequent water uptake lowers both the strength and modulus to less than half their original values. Many investigators have explored both chemistry modifications to the MDF cement mixture and coatings or surface treatments to render the material less hydrophilic.
European Patent EP0585998 B1 discloses a process for improving the moisture resistance through carbonation of a molded item. This process of carbonation requires immersion of an item in a water bath into which carbon dioxide gas is bubbled. This type of process may indeed render the surface of an item less susceptible to degradation but will not provide long-term water resistance to a thick item.
The disclosed cementitious compositions are directed to overcoming one or more of the problems set forth above and other problems of the prior art.